The invention relates to reducing the effects of ionic memory in ferroelectric liquid crystal (FLC) materials.
Liquid crystal devices incorporating a ferroelectric smectic liquid crystal material (FLCDS) are particularly suitable for use in displays and shutters in which their fast switching times and memory characteristics are of advantage. Because of the bistable nature of the material, it is not necessary to continue to apply an electric field to the material in order to maintain the material in a given switched state. A conventional FLCD comprises a layer of ferroelectric smectic liquid crystal material between two parallel glass substrates, electrode structures being typically provided on the inside facing surfaces of the glass substrates in the form of row and column electrodes (on opposite sides of the liquid crystal material) which cross one another to form a matrix array. The intersections of the electrodes define an array of pixels within the material. As is well known, pulses are applied to the row and column electrodes in order to produce electric fields which switch the molecules within the material between two polar states having different molecular orientations. As a result of the different light transmitting properties of the two molecular orientations, when the material is disposed between two polarizers having polarizing axes which are arranged transversely to one another, a display element or pixel at the intersection of two electrodes will appear dark or light depending on the state to which the molecules of the pixel have previously been switched by the electric field due to the voltage difference between the pulses applied to the relevant row electrode on one side of the layer and the relevant column electrode track on the other side of the layer.
Some prior art documents will now be discussed. P Maltese. J. Dijon, T. Leroux, D. Sarrasin, Ferroelectrics (1988), vol. 85, p26s discusses the use of blanking periods containing multiple up and down voltage pulses to provide an improved erasure of previous states. It mentions the problem of using blanking pulses with large Vxc2x7t products of the same polarity. The opposite polarity pulses substantially cancel out any ionic build-up, however both polarities cause full switching of the FLC and the total blanking period is impracticably long. It does not discuss any modification of either the switching pulse itself or the time periods following it.
British Patent No. 2 262 831 seeks to speed up the addressing time of FLCDS whilst improving their contrast ratio. It describes extending the strobe (row or scan) voltage beyond the select period of the row to which it is applied, so as to temporally overlap with the select period of the strobe voltage applied to subsequent row or rows. Regards these extended strobe waveforms it states that xe2x80x9cEach strobe pulse may be immediately followed by a pulse of opposite signxe2x80x9d, no reason for this is given, although in Table 15 a result is given that shows for two field operation of Malvern 2.1 a larger operating window is obtained than with Malvern 2.0.
M. Nitta, N. Ozaki, H. Suenaga, K. Nakaya and S. Kobayashi xe2x80x98Electrooptic Characteristics of a Charge-Transfer Complex-Doped Surface-Stabilized Ferroelectric Liquid Crystal Devicexe2x80x99 Japanese Journal of Applied Physics (1988) vol. 27, no. 4 pL477-L478 describes an improvement of bistability and response speed obtained by doping an FLC material with a charge-transfer complex. The FLC material showed a minimum in response time for both the doped and undoped cases. In the case of the doped material the device exhibited only reverse switching to a single polarity pulse which was faster than forward switching in the undoped material. There is no discussion on the effect this could have on analogue or digital grayscale.
K. Nakaya, B. Y. Zhang, M. Yoshida, I. Isa, S. Shindoh and S. Kobayashi xe2x80x98Electrooptic Bistability of a Ferroelectric Liquid Crystal Device Prepared Using Charge-Transfer Complex-Doped Polyimide-Orientation Filmsxe2x80x99 Japanese Journal of Applied Physics (1989) vol. 28, pL116-L118 describes an enhancement of bistability and response speed obtained by doping the polyimide alignment layer in an FLC device with charge transfer complex. The FLC material used is the same as in (iii) and therefore shows a minimum in response time. The improvement is attributed to neutralization of accumulated surface charge from the spontaneous polarization. There is no discussion on the effect this could have on analogue or digital grayscale.
K. Nakaya, B. Y. Zhang, M. Yoshida, I. Isa, S. Shindoh and S. Kobayashi xe2x80x98Electrooptic Bistability of a Perroelectric Liquid Crystal Device Prepared Using Charge-Transfer Complex-Doped Polyimide-Orientation Filmsxe2x80x99 Japanese Journal of Applied Physics (1989), vol. 28 pL116-L118 describes an enhancement of bistability and response speed obtained by doping the polyimide alignment layer in an FLC device with charge transfer complex. The FLC material used is the same as in (iii) and therefore shows a minimum in response time. The improvement is attributed to neutralization of accumulated surface charge from the spontaneous polarization. There is no discussion on the effect this could have on analogue or digital grayscale.
J. R. Hughes and F. C. Saunders xe2x80x98Inversion of contrast in ferroelectric smectic C liquid crystal displaysxe2x80x99 Liquid Crystals (1988), vol. 3 p1401-1410 describes two switching regimes when using two slot bipolar multiplex waveforms. At lower voltages and longer slot times the switching occurs due to the trailing pulse while at higher voltages and shorter slot times the switching occurs due to the leading pulse. The strobe waveform during the line address period is made up of equal and opposite pulses and no pulses are extended beyond this period.
P. W. H. Surguy, et al. Ferroelectrics (1991), vol. 122, p63 describes the JOERS/Alvey addressing scheme for FLC displays with xcfx84-V min characteristics.
Japanese Patent Publication No. 215616/1992 describes a method to prevent reverse switching by applying an opposite polarity voltage pulse after the first applied pulse.
It is well documented that the performance of ferroelectric liquid crystal devices is significantly affected by ionic contamination: Firstly, ionic response to the internal dipole field causes memory of previous states. This is a particular problem when stable analogue gray levels, which utilize the partial switching behavior of the FLCD, are sought. Ionic memory can also cause problems when temporal dither is used to achieve gray levels because the transmission level in each subframe will depend on the previous subframes even if only the two bistable states are addressed.
Secondly, ionic response to an external applied field destabilizes the switched state and can cause reverse switching after field removal (Ionic Field Latching Effect). If this behavior occurs following a strobe pulse then the operating drive window is cut off by the switching resultant failing to switch at longer pulse widths. If this behavior occurs following a blanking pulse then the operating window is reduced because it causes the non-switching resultant to switch at a lower threshold.
The solution to the first of these problems might be to compensate for previous state dependence by appropriate modification of the data. This requires a real-time computation of the data required to achieve a particular gray level as well as a means of storing the switch history of each pixel. If the dependence on previous states accumulates over several frame times this could require multiple frame stores and becomes impractical to both evaluate and implement. Another solution to this problem is to use longer duration and larger amplitude blanking pulses to reset the ionic field to a uniform state (see prior art (i)). Both of these solutions are limited by the response time of the ions to both external fields and the internal field from the spontaneous polarization. In order to make practical either of these solutions or to resolve the problem of ionic memory altogether it is necessary to reduce the ionic response time. A method to do this is to introduce into the FLC layer a concentration of ionic species which have a faster response time than the intrinsic impurities found in these devices.
One limitation of adding ionic species to the FLC device is that the second of the problems described above becomes worse (i.e. there is a direct correlation with the ionic response time and the threshold for reverse switching). Here we show that this problem can be reduced by applying an opposite polarity voltage pulse immediately after the switching pulse hereafter referred to as the TRIFLE (Technique to Reduce the Ionic Field Latching Effect) pulse. This pulse acts to reduce the ionic reversal field built up by the switching pulse but is carefully optimized so that it does not itself destabilize the switched state.
In order to completely erase the ionic field built up by the switching pulse, a TRIFLE pulse would need to have an equivalent Vxc2x7t product (assuming linearity in the ionic field response). In the simplest model of FLC switching (which considers the ferroelectric torque alone) switching time is inversely proportional to the applied field. Therefore equivalent Vxc2x7t products would be expected to cause the same amount of switching and the TRIFLE pulse would always destabilize the switched state. There is however a significant elastic torque which acts to oppose the switching due to the TRIFLE pulse if the TRIFLE pulse is applied immediately after the switching pulse.
The nature of the chevron layer structure within FLC cells results in different director profiles for field on and field off states. When a field is applied the majority of liquid crystal directors can reorient to a fully switched position. However due to pinningxe2x80x9d of directors at the surfaces and chevron interface this switched state is elastically stressed and relaxation takes place after field removal. Since the starting position of the director profile affects the switching time (i.e. it is much easier to switch from a relaxed state than from a fully switched state (see FIG. 3 and the paper by Hughes and Saunders mentioned above)) the duration of TRIFLE pulse which doesn""t switch can be longer if applied immediately after the switching pulse than if applied to an unstressed state.
This technique has been employed in Japanese Patent Publication No 4-215616 mentioned above to overcome reverse switching from the strobe pulse due to the intrinsic ionic impurities. However in this invention we intend to significantly lower the reverse switching threshold by introducing a large concentration of ions into the FLC layer. When this is done the Vxc2x7t product of the TRIFLE pulse must be similar to the Vxc2x7t product of the strobe pulse which produced the unwanted ionic field. Due to the significantly larger ionic field from the blanking and strobe pulses we require a torque greater than the elastic torque to prevent switching from the larger TRIFLE pulses. This can be achieved by using materials with large dielectric biaxiality.
The dielectric biaxiality of some FLC materials results in a characteristic minimum in the switching time versus voltage curve (FIG. 3a). At high fields this dielectric torque becomes significant and initially opposes the ferroelectric switching torque. Therefore if the amplitude of the TRIFLE pulse is greater than the switching pulse, it is possible to apply equivalent Vxc2x7t products whereby the switching pulse is above the switching curve (FIG. 3(a) circle) and the TRIFLE pulse is below it (FIG. 3(a) cross). (Note that the ionic reversal field built up by the switching pulse constructively adds to the TRIFLE pulse amplitude thus helping to increase the opposing dielectric torque).
The combined effect of the elastic and dielectric torques provides a window of pulses which can be applied to erase the ionic field built up by the switching pulse or blanking pulse without destabilizing the switched state. This is especially useful when the ionic concentration is doped to be significantly larger than the intrinsic ionic concentration of FLC devices.
It is proposed to address fast reverse switching FLC devices which exhibit xcfx84-Vmin characteristics by applying a TRIFLE pulse after either the blanking pulse or the switching pulse or both where the magnitude of this TRIFLE pulse is greater in magnitude than 0.5xc3x97Vs and preferably xe2x89xa71xc3x97Vs where Vs is the magnitude of the strobe voltage. Fast reverse switching is defined as xcfx84100%rev/xcfx840%sw  less than 50 and preferably  less than 30 at some voltage below Vmin where xcfx84100%rev is the minimum duration of a monopolar voltage pulse required to achieve 100% reverse switching and xcfx840%sw is the duration of a monopolar voltage pulse at which forward switching begins (i.e. nucleation of domains). In particular these fast reverse switching devices and addressing means are applied to situations where grayscale is obtained by temporal dither and or analogue means in order to reduce the error in the gray levels due to ionic memory.
According to the invention there is provided a method of reducing the effects of ionic memory in a ferroelectric liquid crystal (FLC) material to which a switching pulse is applied, the method comprising the steps of:
a) adding an ionic dopant to the FLC material, the ionic dopant providing ions having a fast response to an applied electric field; and
b) following said switching pulse by a first pulse of opposite polarity to said switching pulse, in such a way that said first pulse reduces any ionic reversal field created by said switching pulse, but does not destabilize the state to which the FLC material is switched by said switching pulse.
In one embodiment of the invention said ions introduced by said ionic dopant have a faster response time than ions in the FLC material due to intrinsic impurities therein.
Preferably, said FLC material is a xcfx84-V min material, where the minimum duration of a monopolar voltage pulse which achieves forward switching of said FLC material occurs when the amplitude of said monopolar voltage pulse is Vmin.
Preferably, xcfx84100%rev/xcfx840%sw  less than 50 at a voltage below Vmin, where xcfx84100%rev is the minimum duration of a monopolar voltage pulse required to achieve 100% reverse switching of said FLC material containing said dopant, and xcfx840%sw is the duration of a monopolar voltage pulse at which forward switching of said FLC material containing said dopant begins.
Most preferably, xcfx84100%rev/xcfx840%sw  less than 30 at a voltage below Vmm.
In a further embodiment of the invention, said first pulse immediately follows said switching pulse. In a further embodiment of the invention, the Vt product of said first pulse (being the product of the amplitude and duration thereof is substantially equivalent to the Vt product of said switching pulse.
In a further embodiment of the invention, the amplitude of said first pulse is greater than half the amplitude of said switching pulse. In a further embodiment of the invention, the amplitude of said first pulse is greater than the amplitude of said switching pulse.
In a further embodiment of the invention, the amplitude and duration of said first pulse are such that said first pulse lies below the stressed switching curve (as herein defined) of the FLC material containing said dopant.
Said switching pulse may be a blanking or strobe pulse. Alternatively, said first pulse may follow both blanking and strobe pulses.
The invention also provides a ferroelectric liquid crystal (FLC) cell comprising a layer of FLC material enclosed between two substrates and addressed by at least one switching electrode, for applying a switching pulse, and at least one data electrode, for applying a data pulse, wherein the effects of ionic memory in the FLC material are reduced by carry out the method described above.
The invention also provides a light modulating device comprising a FLC cell as described above, and addressing circuitry for addressing said switching and data electrodes.
The invention also provides a ferroelectric liquid crystal (FLC) device comprising a xcfx84-V min FLC material, at least one switching electrode, for applying a switching pulse, and at least one data electrode, for applying a data pulse, wherein:
a) said FLC material is such that at voltages below Vmin, xcfx84100%rev/xcfx840%sw  less than 50, where xcfx84100%rev is the minimum duration of a monopolar voltage pulse required to achieve 100% reverse switching of said FLC material, and xcfx840%sw is the duration of a monopolar voltage pulse at which forward switching of said FLC material begins; and
b) said switching pulse is followed by a first pulse of opposite polarity to said switching pulse.
At voltages below Vmin, xcfx84100%rev/xcfx840%sw may be less than 30.
The invention also provides a light modulating device comprising a FLC device as described above, wherein said FLC material is in the form of a layer, said switching electrode is one of a plurality of such switching electrodes on one side of said layer, said data electrode is one of a plurality of such data electrodes on the other side of said layer, and a plurality of pixels are defined in said layer at the intersections of said switching and data electrodes.